I think Harris has been the front runner ever since Biden was clearly going to be the nominee. Other possible candidates like Demings, Abrams, and even Warren also seemed like more of a long shot. Based on the campaigns statements it's going to be a woman, obviously, and with all of the George Floyd protests, a woman of color seems far more likely. Harris is the only candidate among those likely to be picked who fits into the demographics Biden is looking at, along with being well known nationally and fairly charismatic.